A robot-controlled optical sensor for measuring one or more measurement areas of workpieces in the production environment, for example, on car bodies, is known from U.S. Pat. No. 6,321,137 B1. The optical sensor is mounted on the robot arm and is positioned relative to the workpiece in selected measurement areas with the aid of the robot. The utilization of an optical sensor provides the advantage that such a sensor measures in a contactless fashion and therefore has a substantially faster measuring speed and a lower sensitivity to vibrations than a tactile sensor. This makes it possible to carry out fast and robust measurements in the production environment. The positioning of the optical sensor with the aid of a robot provides the additional advantage of a high flexibility and reduced costs; the positioning with the aid of a robot also makes it possible to achieve an adequate reproducibility and accuracy of the measurement results.
However, this can only be realized if a highly accurate calibration of the complete system—consisting of the robot and the optical sensor—is carried out prior to the actual measuring operation. In this case, it is common practice to initially calibrate the robot by determining its axial error and compensating this axial error with the aid of the control system. In the calibration of the optical sensor system, the error of the sensor optics is compensated and the position of the sensor coordinate system relative to an external reference point, for example, the sensor housing, is determined. It is also necessary to relate the position of the sensor coordinate system to the position of the robot coordinate system in order to determine the position of measuring points of the sensor in the robot coordinate system.
In this context, U.S. Pat. No. 6,321,137 B1 proposes to move the optical sensor into different positions in space relative to a reference body with the aid of the robot and to carry out a calibration of the complete system based on the data from sensor measurement of the reference body obtained in these spatial positions. In this case, however, only a relatively low accuracy is empirically achieved that does not suffice for many applications in the production environment, particularly for measurements on car bodies (shells).